Using recently published infrared photometry of RR Lyrae stars from both the field (Fernley et al. 1992) and globular clusters (Longmore et al. 1990), period shifts have been calculated using the mean (V – K) colour rather than the mean (B – V) colour employed in most previous analyses (e.g., Sandage 1990 and references therein). The advantage of using (V – K) rather than (B – V) is the reduced sensitivity to, firstly, metallicity and, secondly, non-LTE radiation. This latter occurs in many RR Lyrae stars near maximum light and clearly should not be included when calculating the mean colour and hence, mean temperature. This, and all other aspects of the present paper, are discussed more fully in Fernley (1992a).
The mean (V – K) colour of both the field and cluster stars were de-reddened using the maps of Burstein and Heiles (1982) and converted to effective temperature using the calibration of Fernley (1989).