The value of using pulse timing to measure orbital variations in eccentric pulsar binaries which are close enough at periastron to interact tidally is discussed. We briefly review a model for dynamical tidal evolution which shows that chaotic orbital evolution is possible, and list several applications to real systems. The system PSR J0045-7319 is then discussed, and a possible explanation for its decreasing orbital period is suggested.

We propose a new way to measure accurate distances and transverse velocities for some nearby binary pulsars. In many cases the distances will be more accurately determined than is possible by annual parallax, as the relative error decreases as t−5/2. We also note that tests of the general relativistic prediction of orbital period decay of nearby relativistic binary pulsars will be limited to accuracies of a few percent. Nevertheless, PSR B1534+12 observations are consistent with general relativistic predictions if the proper-motion contribution to the orbital period derivative is accounted for.

We report on the discovery and timing observations of the new eclipsing binary millisecond pulsar J2051–0827. This 4.5ms pulsar is in a 2.4 hour binary orbit with a 0.027 M⊙ companion. The companion and the pulsar are separated by only 1 R⊙. The pulsar is often, but not always, eclipsed for approximately 10% of the orbit at frequencies below 660 MHz. This indicates that the eclipse material extends well beyond the Roche lobe. During a number of observations pulses have been detected throughout the eclipse region at 1.4 GHz and 2.0 GHz. The delays measured with respect to the timing model associated with these observations indicate significant structure and variability in the electron column density in the eclipse region.

We discuss a twist in binary neutron star evolution scenarios in which a millisecond pulsar system is actually a progenitor for a low-mass X-ray binary system. After 7 billion years of orbital decay due to gravitational radiation, the millisecond pulsar PSR J0751+1807 will enter a low-mass X-ray binary phase. When the orbital period reaches about 5 minutes, the white dwarf will overflow its Roche lobe, transferring mass to the neutron star. Its predicted observational X-ray parameters are very similar to the ultra-compact low-mass X-ray binary 1820–303. The ultimate fate of the system after Roche-lobe overflow is unknown. One possibility is that the companion may eventually tidally disrupt, leaving a disk of material which could form into planets.

Eclipsing binary pulsars systems highlight an important stage in the evolution of isolated millisecond pulsars. In these systems, the pulsar’s companion is losing mass due to Roche lobe overflow and/or a stellar wind induced by intercepted energy from the pulsar flux. Eventual evaporation of the companions could yield isolated millisecond puslars. PSR B1744-24A was the second eclipsing millisecond pulsar to be discovered (Lyne et al. 1990). It is in a 1.8 hr orbit with a ∼ 0.1 M⊙ companion. Its eclipses show some variability, and the pulsar is undetectable at about 25% of observing epochs, presumably because it is completely enveloped by the companion’s outflow (Nice & Thorsett 1992).

Previous timing data for PSR B1620–26 were consistent with a second companion mass m2 anywhere in the range ∼ 10−3 – 1M⊙, i.e., from a Jupiter-type planet to a star. We present the latest timing parameters for the system, including a significant change in the projected semi-major axis of the inner binary, a marginal detection of the fourth time derivative of the pulse frequency, and the pulsar proper motion (which is in agreement with published values for the proper motion of M4), and use them to further constrain the mass m2 and the orbital parameters. Using the observed value of , we obtain a one-parameter family of solutions, all with m2 ≲ 10−2 M⊙, i.e., excluding stellar masses. Varying within its formal 1σ error bar does not affect the mass range significantly. However, if we vary within a 4σ error bar, we find that stellar-mass solutions are still possible. We also calculate the predicted rate of change of the projected semi-major axis of the inner binary and show that it agrees with the measured value.

Radio timing observations of the SMC binary pulsar PSR J0045–7319 show, for the first time in a binary pulsar, that classical spin-orbit coupling and periastron advance are occurring. From the observations, the angle θ between the B star spin axis and the orbital angular momentum is constrained to be 25° < θ < 41° (Kaspi et al. 1996b). Under the conventional assumption that the pre-supernova angular momenta were aligned, this provides direct evidence for an asymmetric supernova explosion. In addition, a large, unexpected orbital decay is observed in the system.

The origin and evolution of neutron star magnetic fields has been hotly debated for a long time. Spontaneous field decay was originally proposed with timescales of (5–10) × 106 years, while another possible model which associates field decay with mass accretion in the evolution of binary systems has been suggested (see Bhattacharya & van den Heuvel 1991 for a review). The aim of this paper is to examine whether accretion-induced field decay can reproduce the observed properties of the wide binary radio pulsars in quantitative calculations.

In a binary system consisting of a neutron star and a low-mass giant companion, if the initial orbital period is longer than 1 day, mass transfer, taking the form of Roche-lobe overflow, is driven by the nuclear evolution of the giant through radius expansion (Webbink et al. 1983). We assume the mass accretion rate Ṁ of the neutron star is limited to the Eddington accretion rate ṀE ≃ 10−8M⊙ yr−1. If the mass transfer rate is in excess of ṀE, the rest mass is blown from the system in the forms of jets or beams.

The final stage of mass transfer in a compact binary system is very important for calculating the orbital parameters of the resulting system. We demonstrate how simple ‘bifurcation mechanisms‘ may lead to the existence of three classes of binary millisecond pulsars and a possible gap in the orbital period distribution. Recent discoveries indicate that such a gap could exist (see Fig. 2).

Furthermore, we demonstrate that the expected theoretical correlation (e.g. Joss et al. 1987, Rappaport et al. 1995) between orbital periods and white dwarf companion masses does not seem to fit observations of wide-orbit lowmass binary pulsars (LMBPs). It is interesting to notice that the five LMBPs with Porb > 100d all have companion masses which are lighter (at the ∼80 % confidence level on average) than expected from the theoretical core-mass period relation.

Accretion heats the neutron star crust thereby decreasing the electric conductivity in the solid region. This leads to a rapid field decay when the currents supporting the field are concentrated in the solid crust (Geppert & Urpin 1994, Urpin & Geppert 1995). Depending on the duration of the accretion phase and on the total mass accreted the field can be decreased by 3 − 4 orders of magnitude after 106 − 107 years. This mechanism explains the low magnetic fields of many pulsars entering binary systems.

However, there exist both low–mass (e.g. Her X–1, 4U 1626–67) and high–mass (e.g. Cen X–3, SMC X–1) systems, where the neutron star deserves strong accretion and the magnetic field is still large.

Approximately 4/5 of the ∼ 35 millisecond pulsars known in the disk of the Galaxy are in binary systems. The vast majority of these binary pulsars have (presumed) helium white dwarf companions with masses m2 < 0.45 M⊙, spin periods P < 10 ms, and all are in extremely circular orbits (Fig. 1). In a search for millisecond pulsars with the Arecibo radio telescope, we have recently discovered PSR J1022+1001, a 16.45 ms pulsar in a 7.8 d orbit with a companion that is at least 0.73 M⊙, and is more likely as massive as 0.8−1.0M⊙; and PSR J0621+1002, a 28.85 ms pulsar in an 8.3 d orbit with a companion at least 0.45 M⊙, and more likely with m2 ≈ 0.54 M⊙. One other system, PSR J2145–0750, has P = 16.05 ms, orbital period Pb = 6.8 d, and m2 ≈ 0.50 M⊙ (see Table 1).

We use N-body simulations of open clusters to examine the evolutionary history of binaries containing neutron stars. The model includes consistent treatment of all the relevant processes (Aarseth 1996):

1. • Synthetic stellar evolution

2. – mass and radius as a function of time (Eggleton et al. 1989)

3. • Accurate treatment of close encounters

   1. – Perturbations of binaries and stable multiple systems - Exchange reactions (involving triple and binary-binary interactions) - Collisions → blue stragglers

4. • Binary evolution (primordial and dynamically formed)

5. – Tidal interactions (Mardling & Aarseth 1996) - Stable mass transfer - Common envelope evolution - Magnetic braking - Gravitational radiation - Mergers (including blue stragglers)

6. • Mass loss

7. – Stellar winds - Supernovae - Evaporation of stars from the cluster

8. • Neutron star formation

9. – Supernovae with kick velocities of Lyne & Lorimer (1994), Drukier (1996) - Accretion-induced collapse

More than 25 years after the discovery of pulsars, and fifteen years since the first millisecond pulsar was found, we are still unravelling the connection between radio pulsars and the neutron stars in X-ray binaries. In particular, the high binary fraction amongst millisecond pulsars indicates a close relationship with the (low mass) X-ray binaries, and places a premium on the understanding of binary evolution. At the time of the Sydney conference, many issues relating to the evolutionary paths travelled by the millisecond pulsars were subject to controversy, with claims and direct counter-claims evident in the literature. In an attempt to clarify some of these questions, the organisers scheduled one morning of the conference for open debate on “The origin and evolution of millisecond pulsars”.

According to the standard model, millisecond pulsars are the descendants of low-mass X-ray binaries (LMXB). The importance of this formation route has, however, been questioned by several authors on different grounds. This paper critically reviews the arguments and assumptions underlying the standard model. The kinematic properties of the LMXB and millisecond pulsar populations are compared, and are found to be compatible. This provides an additional argument in favour of the standard model.