  The carbon-to-oxygen (C/O) ratio of asymptotic giant branch (AGB) stars
constitutes an important index of evolutionary and environment/metallicity
factor. We develop a method for mass C/O classification of AGBs in photometric
surveys without using periods. For this purpose we rely on the slopes in the
tracks of individual stars in the colour-magnitude diagram. We demonstrate that
our method enables the separation of C-rich and O-rich AGB stars with little
confusion. For the Magellanic Clouds we demonstrate that this method works for
several photometric surveys and filter combinations. As we rely on no period
identification, our results are relatively insensitive to the phase coverage,
aliasing, and time-sampling problems that plague period analyses. For a
subsample of our stars, we verify our C/O classification against published C/O
catalogues. With our method we are able to produce C/O maps of the entire
Magellanic Clouds. Our purely photometric method for classification of C- and
O-rich AGBs constitutes a method of choice for large, near-infrared photometric
surveys. Because our method depends on the slope of colour-magnitude variation
but not on magnitude zero point, it remains applicable to objects with unknown
distances.
