Infrared photometry enables radii, and hence absolute magnitudes, to be derived more accurately than is possible using purely optical photometry. In this paper we present BVJHK observations of the galactic cepheids T Vul, U Vul and T Mon. Using this data we find reasonable agreement with current versions of the Period-Luminosity relations, in both the optical and infrared.
From these relations and existing optical and infrared data for LMC Cepheids we find for the LMC (i) Av = 0.35 and (ii) a distance modulus of 18.50. In addition we show that the P-L-C relation has the form
<Mv> = – 3.10 log P + 1.70 (<B>–<V>) – 2.37