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Published online by Cambridge University Press: 04 August 2017
Lockwood's measurements in five-color narrow-band photometry of spectroscopic standard M stars is used to define an estimator of temperature. The TiO and VO indices deduced from this photometry depend in the same way on the metallicity but vary in a different way with the temperature: their ratio is only influenced by the temperature. A relation between T/V and the temperature is calibrated for M giants between 2000 and 3000° K. Both the T and V indices are indicators of metallicity and gravity which allow us to check that the relation (T/V)/temperature is independent of the metallicity. This relation leads to a precise determination of the temperature and can be applied to a wide variety of M giants, independently of their reddening and chemical composition.