On the incidence rate of first overtone Blazhko stars in the Large Magellanic Cloud

Aims. By using the full span of multicolor data on a representative sample of first overtone RR Lyrae stars in the Large Magellanic Cloud (LMC), we revisit the problem of the incidence rate of the amplitude/phase-modulated (Blazhko) stars. Methods. Multicolor data, obtained by the MAssive Compact Halo Objects (MACHO) project, are utilized through a periodogram averaging method. Results. The method of analysis enabled us to increase the number of detected multiperiodic variables by 18%, relative to the number obtained by the analysis of the best single color data. We also test the maximum modulation period detectable in the present dataset. We find that variables showing amplitude/ phase modulations with periods close to the total time span can still be clearly separated from the class of stars showing period changes. This larger limit on the modulation period, the more efficient data analysis and the longer time span lead to a substantial increase in the incidence rate of the Blazhko stars in comparison to earlier results. We find altogether 99 first overtone Blazhko stars in the full sample of 1332 stars, implying an incidence rate of 7.5%. Although this rate is nearly twice the one derived earlier, it is still significantly lower than that of the fundamental mode stars in the LMC. The by-products of the analysis (e. g. star-by-star comments, distribution functions of various quantities, etc.) are also presented.