We report the discovery of a period doubling behaviour in a 2.4 d BL Herculis-type variable of the Galactic bulge .
Another bulge BL Her-type star ( P = 2.25 d ) 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 ( 8 ) .
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 = 2.4 d. 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 \approx 0.01 .
The recent evolutionary tracks put also constraint on the mass of the star , which is close to M = 0.50 { } { M } _ { \odot } .