We investigate the reddening and Main Sequence fitted distances to eleven 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 .
By main-sequence fitting we proceed to calibrate the Cepheid P-L relation and find M _ { V } =-2.81 \times logP-1.33 \pm 0.32 and M _ { K } =-3.44 \times logP-2.20 \pm 0.29 and a distance modulus to the LMC of 18.55 \pm 0.32 in the V -band and 18.47 \pm 0.29 in the K -band giving an overall distance modulus to the LMC of 18.51 \pm 0.3 .

In the case of two important clusters we find that the U-B : B-V diagram in these clusters is not well fitted by the standard Main Sequence line .
In one case , NGC7790 , we find that the F stars show a UV excess which if caused by metallicity would imply Fe/H \sim -1.5 ; this is anomalously low compared to what is expected for young open clusters .
In a second case , NGC6664 , the U-B : B-V diagram shows too red U-B colours for the F stars which in this case would imply a higher than solar metallicity .
If these effects are due to metallicity then it would imply that the Cepheid PL ( V ) and PL ( K ) zeropoints depend on metallicity according to \frac { \delta M } { \delta Fe / H } \sim 0.66 in the sense that lower metallicity Cepheids are intrinsically fainter .
Medium-high resolution spectroscopy for the main-sequence F stars in these two clusters is needed to determine if metallicity really is the cause or whether some other explanation applies .

Please see http : //star-www.dur.ac.uk:80/ fhoyle/papers.html for a version with all the figures correctly inserted .