  By analysing the K2 short-cadence data from Campaign 14 we detect phase-curve
modulation in the light curve of the hot-Jupiter host star WASP-104. The
ellipsoidal modulation is detected with high significance and in agreement with
theoretical expectations, while Doppler beaming and reflection modulations are
detected tentatively. We show that the visual geometric albedo is lower than
0.03 at 95% confidence, making it one of the least-reflective planets found to
date. The light curve also exhibits a rotational modulation, implying a stellar
rotational period likely to be near 23 or 46 days. In addition, we refine the
system parameters and place tight upper limits for transit timing and duration
variations, starspot occultation events, and additional transiting planets.
