Galactic orbits of 519 type II Cepheids classified as BL Her, W Vir, and RV Tau variable stars are calculated using Bayesian distances, proper motions, and Gaia DR3 radial velocities. At least 27 BL Her variables out of 100, 17 W Vir out of 308, and 8 RV Tau out of 111 have space positions and orbits typical of thin/old disk population of the Galaxy. At the same time, their luminosities are completely consistent with that of type II Cepheids. The stars at different stages of evolution can coexist in the instability strip of BL Her type Cepheids: (i) metal-deficeint, low-mass halo and thick disc stars; (ii) primary components of some spectroscopic binary Cepheids considered as thin/old disc stars (AU Peg and possibly TX Del) that have undergone mass exchange with sattelite; (iii) initially more massive single thin/old disk stars which could have enough time to evolve to the onset of core helium burning in less than 8–10 Gyr; their evolutionary ‘‘loops’’ explain the existence of a number of BL Her Cepheids demonstrating an evolutionary decrease of pulsation period. Detailed pulsation calculations and a comprehensive study is needed to refine the classification of population II Cepheids.
Rastorguev et al. (Mon,) studied this question.