Published online by Cambridge University Press: 14 August 2015
The distribution of long-period Population I cepheids is studied. An age comparison of cepheids and galactic clusters shows that cepheids with periods > 11.25 d, corresponding to ages of ≲ 30 × 106 yr, should be almost as good spiral arm tracers as galactic clusters with earliest types b 2–3. The distances of cepheids are determined from a revised period-luminosity-colour relation and from colour excesses, for the determination of which a new, purely photometric method is given. The resulting distribution of cepheids shows a good correlation with the spiral arms, as traced by young clusters.