  We present the discovery and characterization of WASP-148, a new extrasolar
system that includes at least two giant planets. The host star is a slowly
rotating inactive late-G dwarf with a V=12 magnitude. The planet WASP-148b is a
hot Jupiter of 0.72 R_Jup and 0.29 M_Jup that transits its host with an orbital
period of 8.80 days. We found the planetary candidate with the SuperWASP
photometric survey, then characterized it with the SOPHIE spectrograph. Our
radial velocity measurements subsequently revealed a second planet in the
system, WASP-148c, with an orbital period of 34.5 days and a minimum mass of
0.40 M_Jup. No transits of this outer planet were detected. The orbits of both
planets are eccentric and fall near the 4:1 mean-motion resonances. This
configuration is stable on long timescales, but induces dynamical interactions
so that the orbits differ slightly from purely Keplerian orbits. In particular,
WASP-148b shows transit-timing variations of typically 15 minutes, making it
the first interacting system with transit-timing variations that is detected on
ground-based light curves. We establish that the mutual inclination of the
orbital plane of the two planets cannot be higher than 35 degrees, and the true
mass of WASP-148c is below 0.60 M_Jup. We present photometric and spectroscopic
observations of this system that cover a time span of ten years. We also
provide their Keplerian and Newtonian analyses; these analyses should be
significantly improved through future TESS~observations.
