U BV photoelectric photometry of the 1H/2H double mode cepheid CO Aurigae

Abstract

We performed new U BV photoelectric photometry of the double mode cepheid CO Aur. A careful time series analysis, by means of various algorithms, of our light curves of CO Aur enabled us to confirm the two previously known periodicities (Antonello et al. 1986) and their ratio of about 0.800. This ratio can be explained if CO Aur is a double mode cepheid pulsating in the first and second overtone (1H/2H); only two such stars are known in our galaxy. From the analysis of present data, the high stability of the periods stated by Fuhrmann et al. (1984) is fully confirmed; there is no clear evidence of variations of other light curve parameters. Within the limits of our photoelectric data, the secondary periodicity of CO Aur has a perfectly sinusoidal shape. We also performed a time series analysis of the Tycho epoch photometry in B_T and V_T; and the high stability of the first period of CO Aur is confirmed by the Tycho datasets too.