Classical cepheids with and without bumps in the light curves

By use of the photometric catalogue of Nikolov (1968), the presence or the absence of bumps in the light curves of 116 classical cepheids with periods approximately between 6 and 20 days is established and the ratio of the variables without bumps to all the stars in this range in period is determined. In the systems of evolutionary tracks of Iben (1967) and Paczyski (1970), the times for the first three crossings of the instability strip by model stars of different masses are calculated and the ratio of the time for the first crossing to the total time spent within the instability strip (taking into account the three crossings) is determined for different stellar masses and pulsation periods. The comparison of these observational and theoretical data does not support B. F. Madore's suggestion (in connection with the hypothesis of A. N. Cox and collaborators for inhomogeneous cepheid envelopes) that the classicial cepheids without bumps in this period range might cross for the first time the instability strip: the fraction of such variables is considerably larger than the theoretically expected fraction. The absence of bumps might be a consequence of a mixing due to, e.g., a differential rotation and/or the Rayleigh-Taylor instability, or a consequence of differences in luminosity, mass, effective temperature or chemical composition among cepheids of the same periods.