Long-term CCD Photometry and Physical Properties of the sdB+M Eclipsing System 2M 1533+3759

Abstract

New CCD photometry of seven successive years from 2010 is presented for the HW Vir-type eclipsing binary 2M 1533+3759. Using the $VI$ light curves together with the radial-velocity data given by For et al. (2010), we determined the absolute parameters of each component to be $M_1$ = 0.442$\pm$0.012 M$_\odot$, $M_2$ = 0.124$\pm$0.005 M$_\odot$, $R_1$ = 0.172$\pm$0.002 R$_\odot$, $R_2$ = 0.157$\pm$0.002 R$_\odot$, $L_1$ = 19.4$\pm$1.4 L$_\odot$, and $L_2$ = 0.002$\pm$0.002 L$_\odot$. These indicate that 2M 1533+3759 is a detached system consisting of a normal sdB primary and an M7 dwarf companion. Detailed analyses of 377 minimum epochs, including our 111 timings, showed that the orbital period of the system remains constant during the past 12 yrs. Inspecting both types of minima, we found a delay of 3.9$\pm$1.0 s in the arrival times of the secondary eclipses relative to the primary eclipse times. This delay is in satisfactory agreement with the predicted R{\o}mer delay of 2.7$\pm$1.4 s and the result is the second measurement in sdB+M eclipsing binaries. The time shift of the secondary eclipse can be explained by some combination of the R{\o}mer delay and a non-zero eccentricity. Then, the binary star would have a very small eccentricity of $e \cos {\omega} \simeq$ 0.0001.