  Estimates of cosmological parameters using galaxy clusters have the scatter
in the observable at a given mass as a fundamental parameter. This work
computes the amplitude of the scatter for a newly introduced mass proxy, the
product of the cluster total luminosity times the mass-to-light ratio, usually
referred as stellar mass. The analysis of 12 galaxy clusters with excellent
total masses shows a tight correlation between the stellar mass, or stellar
fraction, and total mass within r500 with negligible intrinsic scatter: the 90%
upper limit is 0.06 dex, the posterior mean is 0.027 dex. This scatter is
similar to the one of best-determined mass proxies, such as Yx, i.e. the
product of X-ray temperature and gas mass. The size of the cluster sample used
to determine the intrinsic scatter is small, as in previous works proposing
low-scatter proxies because very accurate masses are needed to infer very small
values of intrinsic scatter. Three-quarters of the studied clusters have lgM
<~14 Msol, which is advantageous from a cosmological perspective because these
clusters are far more abundant than more massive clusters. At the difference of
other mass proxies such as Yx, stellar mass can be determined with survey data
up to at least z=0.9 using upcoming optical near-infrared surveys, such as DES
and Euclid, or even with currently available surveys, covering however smaller
solid angles. On the other end, the uncertainty about the predicted mass of a
single cluster is large, 0.21 to 0.32 dex, depending on cluster richness. This
is largely because the proxy itself has ~0.10 dex errors for clusters of lgM<~
14 Msol mass.
