We present new photometric and spectroscopic observations for 2M 1533+3759 ( = NSVS 07826147 ) , the seventh eclipsing subdwarf B star + M dwarf ( sdB+dM ) binary ever found .
It has an orbital period of 0.16177042 day , or \sim 3.88 h , significantly longer than the 2.3–3.0 hour periods of the other known eclipsing sdB+dM systems .
Spectroscopic analysis of the hot primary yields T _ { eff } = 29230 \pm 125 K , \log g = 5.58 \pm 0.03 and log N ( He ) / N ( H ) = -2.37 \pm 0.05 .
The sdB velocity amplitude is K _ { 1 } = 71.1 \pm 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 magnitudes , 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 86.6 \arcdeg .
The sdB primary mass is M _ { 1 } = 0.376 \pm 0.055 ~ { } M _ { \sun } and its radius is R _ { 1 } = 0.166 \pm 0.007 ~ { } R _ { \sun } .
2M 1533+3759 joins PG 0911+456 ( and possibly also HS 2333+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 _ { \sun } , down to as low as 0.30 ~ { } M _ { \sun } , were theoretically predicted by Han et al .
( 2002 , 2003 ) , 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 _ { \sun } implies that at least some sdB progenitors have initial main sequence masses of 1.8–2.0 ~ { } M _ { \sun } or more , i.e . they are at least main sequence A stars .
The orbital separation in 2M 1533+3759 is a = 0.98 \pm 0.04 ~ { } R _ { \sun } .
The secondary has M _ { 2 } = 0.113 \pm 0.017 ~ { } M _ { \sun } , R _ { 2 } = 0.152 \pm 0.005 ~ { } R _ { \sun } and T _ { eff _ { 2 } } = 3100 \pm 600 K , consistent with a main sequence M5 star .
If 2M 1533+3759 becomes a cataclysmic variable ( CV ) , its orbital period will be 1.6 h , below the CV period gap .