Orbital period vs. log L_X: for STW (1) shown as black points. The magenta and gray ‘tri_down’ symbols represent the X-ray upper limits for some EW-type binaries from XMM-Newton pointed and slew survey, respectively; for SRASS (2) shown in black points. The gray ‘tri_right’ symbols represent the X-ray upper limits for some EW-type binaries from ROSAT survey. The period vs. slopes with their corresponding error bars of STW (black) and SRASS (red) with different limiting period ranges (3). The black dashed lines are at P = 0.44 days. The solid blue lines stand for best-fit linear correlations, and the light blue dashed lines represent 95% uncertainty ranges.