The Temperature and Continuum of the N Stars

On the basis of (R — I) colours it has been suggested that the N stars (the cool carbon stars) occupy a very restricted region in the M _b01 — log T _e diagram (T _e = 3300 to 3500 K) M _b01 —3), falling on the normal giant sequence, but hotter than the M variables. This appeared somewhat puzzling in view of the variability of many of the carbon stars and their large range of periods, which suggested that the N stars ought perhaps to have similar temperatures to the variable M stars. In order to compare the continua of the M stars and N stars within the range of the R and I filters, and also to determine the continuum between the molecular bands, photometric observations were made of 20 M and N stars at carefully chosen wavelengths with 30Å resolution.