Observed instability strip in the plane K_{s, 0} versus (J − K_s)₀. Filled circles, open circles, crosses and stars show BL Her, W Vir, pW Vir and RV Tau variables, respectively. The solid lines show the approximate borders of the BL Her/W Vir instability strip. Blue and red edges are described by the following equations: K_{s, 0} = 19.1−21(J − K_s)₀ (0.06 < (J − K_s)₀ < 0.27 mag) and K_{s, 0} = 27.1−21(J − K_s)₀ (0.44 < (J − K_s)₀ < 0.63 mag), respectively.