  We have computed a three-dimensional hydrodynamic simulation of the merger
between a massive ($0.4\,M_\odot$) helium white dwarf (He WD) and a low-mass
($0.6\,M_\odot$) carbon-oxygen white dwarf (CO WD). Despite the low mass of the
primary, the merger triggers a thermonuclear explosion as a result of a double
detonation, producing a faint transient and leaving no remnant behind. This
type of event could also take place during common-envelope mergers whenever the
companion is a CO WD and the core of the giant star has a sufficiently large He
mass. The spectra show strong Ca lines throughout the first few weeks after the
explosion. The explosion only yields $<0.01\,M_\odot$ of $^{56}$Ni, resulting
in a low-luminosity SN Ia-like lightcurve that resembles the Ca-rich transients
within this broad class of objects, with a peak magnitude of $M_\mathrm{bol}
\approx -15.7\,$mag and a rather slow decline rate of $\Delta
m_{15}^\mathrm{bol}\approx 1.5\,$mag. Both, its lightcurve-shape and spectral
appearance, resemble the appearance of Ca-rich transients, suggesting such
mergers as a possible progenitor scenario for this class of events.
