Distances to Cepheid open clusters via optical and K-band imaging

We investigate the reddening and main-sequence-fitted distances to 11 young, Galactic open clusters that contain Cepheids. Each cluster contains or is associated with at least one Cepheid variable star. Reddening to the clusters is estimated using the U-B:B-V colours of the OB stars and the distance modulus to the cluster is estimated via B-V:V and V-K:V colour-magnitude diagrams. Our main-sequence fitting assumes that the solar-metallicity zero-age main sequence of Allen applies universally to all the open clusters, although this point is controversial at present. In this way we proceed to calibrate the Cepheid period-luminosity (PL) relation and find M_V=-2.87 × logP- 1.243 +/- 0.09, M_K=-3.44 × logP- 2.21 +/- 0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of 18.54 +/- 0.10 from the V-band and 18.48 +/- 0.10 from the K-band giving an overall distance modulus to the LMC of μ₀= 18.51 +/- 0.10. This is in good agreement with the previous Cepheid PL-K result of Laney & Stobie at μ₀= 18.51 +/- 0.09 and with the Hipparcos parallax-calibrated Cepheid PL-K estimate of Feast & Catchpole at μ₀= 18.66 +/- 0.10 when no account is taken of the LMC metallicity.

We also find that the two-colour U-B:B-V diagrams of two important clusters are not well fitted by the standard main-sequence line. In one case, NGC 7790, we find that the F stars show a UV excess and in the second case, NGC 6664, they are too red in U-B. Previous spectroscopic estimates of the metallicity of the Cepheids in these clusters appear to suggest that the effects are not due to metallicity variations. Other possible explanations for these anomalies are positional variations in the dust reddening law and contamination by foreground or background stars.