Time series of B , V , I CCD photometry and radial velocity measurements from high resolution spectroscopy ( R=30,000 ) covering the full pulsation cycle are presented for the field RR Lyrae star CM Leonis .
The photometric data span a 6 year interval from 1994 to 1999 , and allow us to firmly establish the pulsation mode and periodicity of the variable .
The derived period P =0.361699 days ( \pm 0.000001 ) is very close to the value published in the Fourth Edition of the General Catalogue of Variable Stars ( P =0.361732 days ) .
However , contrary to what was previously found , the amplitude and shape of the light curve qualify CM Leo as a very regular first overtone pulsator with a prominent hump on the rising branch of its multicolour light curves .
According to an abundace analysis performed on three spectra taken near minimum light ( 0.42 < \phi < 0.61 ) , CM Leo is a metal-poor star with metal abundance [ Fe/H ] = -1.93 \pm 0.20 .
The photometric and radial velocity curves of CM Leo have been compared with the predictions of suitable pulsational models to infer tight constraints on the stellar mass , effective temperature , and distance modulus of the star .
We derive a true distance modulus of CM Leo of \mu _ { 0 } =13.11 \pm 0.02 mag and a corresponding absolute magnitude of M _ { V } =0.47 \pm 0.04 .
This absolute magnitude , once corrected for evolutionary and metallicity effects , leads to a true distance modulus of the Large Magellanic Cloud of \mu _ { 0 } =18.43 \pm 0.06 mag , in better agreement with the long astronomical distance scale .