| Summary: | In this thesis complete light curves, at both optical (V) and infrared (J, H, K) wavelengths, are presented for 15 pulsating variable stars. These include four dwarf Cepheids, six RR Lyraes and five Cepheids. In addition, radial velocity curves are also presented for three stars. Using this data, and existing radial velocity curves taken from the literature, we derive radii for all the stars using a variant of the Baade-Wesselink method. The value of the infrared for radius determination is illustrated by the fact that no previous attempts to apply Baade-Wesselink type methods, in an empirical manner, to short period (P <1 day) variables have been successful. From these results the following conclusions are drawn: 1. Metal poor RR Lyraes have ? 0.55 and the metal rich, shorter period variables appear to be ∼0.3 - 0.4 mags fainter. These results and data secured recently for the globular clusters M5 and M107 show that RR Lyraes obey a period-luminosity relation in the infrared of the form = -1.01 - 2.80 log P The Cepheid period-luminosity relation from our data requires that the Cepheid zero point from cluster main sequence fitting should be reduced by ~0.1. With this revision a distance modulus of 18.50 +/- 0.15 for the LMC is given by both RR Lyraes and Cepheids. The "pulsation" masses of both the RR Lyraes and Cepheids are systematically smaller than the "evolutionary" masses by ∼15%. The "pulsation" masses of dwarf Cepheids depend on the assumed pulsation mode. For those stars of known (or suspected) pulsation mode, low masses are derived. If this is generally true then dwarf Cepheids and delta Scutis are dissimilar. Further evidence in support of this conclusion is provided by, firstly, a comparison of the observed pulsation characteristics (periods and amplitudes) of the two groups of stars and, secondly, a comparison of the observed period changes of dwarf Cepheids with the theoretically calculated period changes.
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