Infrared photometry of Mira variables in the Baade windows and the distance to the Galactic Centre

JHKL infrared observations are presented for 70 Mira variables in the Galactic Centre windows, Sgr I and NGC 6522. Distances were derived using the period–luminosity relation of Glass and Lloyd Evans for LMC Miras. The space densities of the Miras show a well marked maximum. The narrowness of the distribution (uncorrected FWHM = 1.9 kpc) is similar to that found by Oort & Plaut for RR Lyrae variables and shows that the Galactic Centre Miras fit a period–luminosity relation with very small scatter ($$\sigma\sim0.2$$ mag). An analysis of the Mira data alone yields $$A_\nu = 2.0$$ mag, R₀ = 8.8 kpc. The data can be combined with the Oorty–Plaut analysis of RR Lyrae variables in the NGC6522 field to yield $$A_\nu = 1.5$$ mag, R₀ = 9.2(± 0.6) kpc which we adopt as our best estimate. Apart from five very red variables whose nature remains uncertain, the Galactic Centre Miras populate the same regions of colour–colour plots as do Miras in the solar neighbourhood.