  The growth of brightest cluster galaxies is closely related to the properties
of their host cluster. We present evidence for dry mergers as the dominant
source of BCG mass growth at $z\lesssim1$ in the XXL 100 brightest cluster
sample. We use the global red sequence, H$\alpha$ emission and mean star
formation history to show that BCGs in the sample possess star formation levels
comparable to field ellipticals of similar stellar mass and redshift. XXL 100
brightest clusters are less massive on average than those in other X-ray
selected samples such as LoCuSS or HIFLUGCS. Few clusters in the sample display
high central gas concentration, rendering inefficient the growth of BCGs via
star formation resulting from the accretion of cool gas. Using measures of the
relaxation state of their host clusters, we show that BCGs grow as relaxation
proceeds. We find that the BCG stellar mass corresponds to a relatively
constant fraction 1\%\ of the total cluster mass in relaxed systems. We also
show that, following a cluster scale merger event, the BCG stellar mass lags
behind the expected value from the M$_{cluster}$ - M$_{BCG}$ relation but
subsequently accretes stellar mass via dry mergers as the BCG and cluster
evolve towards a relaxed state.
