Discovery of period doubling in BL Herculis stars of the OGLE survey. Observations and theoretical models

We report the discovery of a period doubling behaviour in a 2.4d BL Herculis-type variable of the Galactic bulge. Another bulge BL Her-type star (P=2.25d) is a strong period doubling candidate. Both objects have been identified with the OGLE-III photometry. Possibility of period doubling in this type of pulsators has been predicted almost twenty years ago by Buchler & Moskalik. Our finding is the first observational confirmation of their theoretical results. Discovery of the first BL Herculis star showing the period doubling effect motivates a new theoretical investigation with the state-of-the-art convective pulsation codes. We present the results of initial model survey, specifically aimed at studying the observed period-doubled BL Her variable. All of our non-linear models have P=2.4d. The computations confirm that the period doubling effect is caused by the 3:2 resonance between the fundamental mode and the first overtone, as indicated by earlier radiative models of Buchler & Moskalik. Comparison of the computed and the observed light curves allows to constrain the parameters of the star, in particular its metallicity, which appears to be high, Z is approximately 0.01. The recent evolutionary tracks put also constraint on the mass of the star, which is close to M=0.50 solar masses.