Modeling the system parameters of 2M 1533+3759: A new longer period low-mass eclipsing SDB+DM binary

We present new photometric and spectroscopic observations for 2M 1533+3759 (= NSVS07826147), the seventh eclipsing subdwarf B star + M dwarf (sdB+dM) binary ever found. It has an orbital period of 0.16177042days, or ∼ 3.88 hr, significantly longer than the 2.3-3.0 hr periods of the other known eclipsing sdB+dM systems. Spectroscopic analysis of the hot primary yields T _eff = 29230 ± 125 K, log g = 5.58 ± 0.03, and log N(He)/N(H)= -2.37 ± 0.05. The sdB velocity amplitude is K ₁ = 71.1 ± 1.0 km s^-1. The only detectable light contribution from the secondary is due to the surprisingly strong reflection effect, whose peak-to-peak BVRI amplitudes are 0.10, 0.13, 0.15, and 0.19 mag, respectively. Light-curve modeling produced several solutions corresponding to different values of the system mass ratio, q (M ₂/M ₁), but only one is consistent with a core helium burning star, q = 0.301. The orbital inclination is 866. The sdB primary mass is M ₁ = 0.376 ± 0.055 M _⊙ and its radius is R ₁ = 0.166 ± 0.007 R _⊙. 2M 1533+3759 joins PG0911+456 (and possibly also HS2333+3927) in having an unusually low mass for an sdB star. SdB stars with masses significantly lower than the canonical value of 0.48 M _⊙, down to as low as 0.30 M _⊙, were theoretically predicted by Han et al., but observational evidence has only recently begun to confirm the existence of such stars. The existence of core helium burning stars with masses lower than 0.40-0.43 M _⊙ implies that at least some sdB progenitors have initial main-sequence masses of 1.8-2.0 M_ȯ or more, i.e., they are at least main-sequence A stars. The orbital separation in 2M 1533+3759 is a = 0.98 ± 0.04R _⊙. The secondary has M ₂ = 0.113 ± 0.017 M _⊙, R ₂ = 0.152 ± 0.005R _⊙, and K, consistent with a main-sequence M5 star. If 2M 1533+3759 becomes a cataclysmic variable (CV), its orbital period will be 1.6 hr, below the CV period gap.