First four-color photometric investigation of extreme mass-ratio contact binary AS coronae borealis

•The first photometric analysis of AS CrB with quality four-color light curves.•The physical parameters are well estimated due to the totally eclipsing.•Finding the period of AS CrB is long-term increasing.•AS CrB is close to the merge condition.

The first high-precision BVRI   light curves of the eclipsing binary AS CrB were presented and were analyzed by the 2015 version of the W-D code. It is found that AS CrB is an extreme mass-ratio, deep contact binary with a fill-out factor of f=59.6±2.5%f=59.6±2.5% and a mass ratio of q=0.172±0.008q=0.172±0.008. Based on the photometric solution and the Dartmouth isochrones model, the masses, radii, and luminosities of the components are estimated as follows: M1=1.25±0.15M⊙,M1=1.25±0.15M⊙,M2=0.21±0.06M⊙,M2=0.21±0.06M⊙,R1=1.40±0.07R⊙,R1=1.40±0.07R⊙,R2=0.67±0.04R⊙,R2=0.67±0.04R⊙,L1=3.2±0.2L⊙,L1=3.2±0.2L⊙, and L2=0.72±0.04L⊙,L2=0.72±0.04L⊙, with an estimated distance 459 ± 42 pc. These uncertainties mainly come from the errors of the color used to estimate the temperature of the primary star. By investigating all of the available times of light minima, it is found that the Observed-Calculated [(O−C)(O−C)] curve shows a long-term period increase, with a rate of dP/dt=+(3.46±0.01)×10−7dP/dt=+(3.46±0.01)×10−7 day/year. As an extreme mass-ratio contact binary, AS CrB may merge into a single star, such as an FK Com-type star or a blue straggler, because of the orbital instability.