Projection factor and radii of Type II Cepheids - BL Her stars

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Table 4.

Surface brightness-colour relations used to estimate the angular diameters of Type II Cepheids.

Abbreviation	Relation	Calibrating sample	Source
K04a	F_V = − 0.1336 × (V − K)₀ + 3.9530	classical Cepheids	Kervella et al. (2004a)
		0.95 < (V − K_S)₀ < 2.4
K04b	F_V = −0.1377 × (V − K)₀ + 3.9620	dwarfs and subgiants	Kervella et al. (2004b)
		0.5 < (V − K_S)₀ < 3
S21	F_V = −0.1404 × (V − K)₀ + 3.9665	dwarfs and subgiants	Salsi et al. (2021)
		1 < (V − K_S)₀ < 3
G21	$F_{V} = - 0.0031 \times {(V - K)}_{0}^{5} + 0.0239 \times {(V - K)}_{0}^{4}$ $F_V=-0.0031\times (V-K)_0 ^5 +0.0239\times (V-K)_0 ^4$	dwarfs	Graczyk et al. (2021)
	$- 0.0623 \times {(V - K)}_{0}^{3} + 0.0705 \times {(V - K)}_{0}^{2}$ $-0.0623 \times (V-K)_0 ^3+0.0705 \times (V-K)_0 ^2$	0.5 < (V − K_S)₀ < 2.1
	−0.1708 × (V − K)₀ + 3.9666

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