  We report on the discovery of large-amplitude flickering from V648 Car (=
SS73-17), a poorly studied object listed amongst the very few hard X-ray
emitting symbiotic stars. We performed milli-magnitude precision optical
photometry with the Swope Telescope at the Las Campanas Observatory, Chile, and
found that V648 Car shows large U-band variability over time scales of minutes.
To our knowledge, it is amongst the largest flickering of a symbiotic star ever
reported. Our finding supports the hypothesis that symbiotic WDs producing hard
X-rays are predominantly powered by accretion, rather than quasi-steady nuclear
burning, and have masses close to the Chandrasekhar limit. No significant
periodicity is evident from the flickering light curve. The ASAS long-term V
light curve suggests the presence of a tidally distorted giant accreting via
Roche Lobe overflow, and a binary period of about 520 days. On the basis of the
outstanding physical properties of V648 Car as hinted by its fast and long-term
optical variability, as well as by its nature as hard X-ray emitter, we
therefore call for simultaneous follow-up observations in different bands,
ideally combined with time-resolved optical spectroscopy.
