The peculiar sdB NGC6121-V46: A low-mass double degenerate ellipsoidal variable in a globular cluster

The variable sdB known as V46 in the globular cluster M4 has remained enigmatic since its discovery almost 10 years ago. We present here radial velocity measurements obtained from medium-resolution VLT/FORS2 spectra that show variations at twice the period of the luminosity changes. This implies that the system is an ellipsoidal variable. Unlike the other sdB binaries of this nature, the fundamental parameters of this star we derive suggest that it lies below the Zero Age Extreme Horizontal Branch. From the cluster distance and the gravity we determine the mass of V46 to be ~0.19Msun. This is too low to sustain core helium burning. From the mass function we derive a lower limit for the companion of only 0.26Msun. We discuss the star's origin in the context of close binary evolution in the field and globular clusters.