A ground-based measurement of the relativistic beaming effect in a detached double white dwarf binary

We report on the first ground-based measurement of the relativistic beaming effect (aka Doppler boosting). We observed the beaming effect in the detached, non-interacting eclipsing double white dwarf (WD) binary NLTT 11748. Our observations were motivated by the system's high mass-ratio and low-luminosity ratio, leading to a large beaming-induced variability amplitude at the orbital period of 5.6 hr. We observed the system during three nights at the 2.0 m Faulkes Telescope North with the SDSS-g′ filter and fitted the data simultaneously for the beaming, ellipsoidal, and reflection effects. Our fitted relative beaming amplitude is (3.0 ± 0.4) × 10^-3, consistent with the expected amplitude from a blackbody spectrum given the photometric primary radial velocity (RV) amplitude and effective temperature. This result is a first step in testing the relation between the photometric beaming amplitude and the spectroscopic RV amplitude in NLTT 11748 and similar systems. We did not identify any variability due to the ellipsoidal or reflection effects, consistent with their expected undetectable amplitude for this system. Low-mass, helium-core WDs are expected to reside in binary systems, where in some of those systems the binary companion is a faint C/OWD and the two stars are detached and non-interacting, as in the case of NLTT 11748. The beaming effect can be used to search for the faint binary companion in those systems using wide-band photometry.