  We report the discoveries of three transiting exoplanets. WASP-91b is a warm
Jupiter (1.34 $M_{\rm Jup}$, 1.03 $R_{\rm Jup}$) in a 2.8-day orbit around a
metal-rich K3 star. WASP-105b is a warm Jupiter (1.8 $M_{\rm Jup}$, 0.96
$R_{\rm Jup}$) in a 7.9-day orbit around a metal-rich K2 star. WASP-107b is a
warm super-Neptune/sub-Saturn (0.12 $M_{\rm Jup}$, 0.94 $R_{\rm Jup}$) in a
5.7-day orbit around a solar-metallicity K6 star. Considering that giant
planets seem to be more common around stars of higher metallicity and stars of
higher mass, it is notable that the hosts are all metal-rich, late-type stars.
With orbital separations that place both WASP-105b and WASP-107b in the
weak-tide regime, measurements of the alignment between the planets' orbital
axes and their stars' spin axes may help us to understand the inward migration
of short-period, giant planets.
  The mass of WASP-107b (2.2 $M_{\rm Nep}$, 0.40 $M_{\rm Sat}$) places it in
the transition region between the ice giants and gas giants of the Solar
System. Its radius of 0.94 $R_{\rm Jup}$ suggests that it is a low-mass gas
giant with a H/He-dominated composition. The planet thus sets a lower limit of
2.2 $M_{\rm Nep}$ on the planetary mass above which large gaseous envelopes can
be accreted and retained by proto-planets on their way to becoming gas giants.
We may discover whether WASP-107b more closely resembles an ice giant or a gas
giant by measuring its atmospheric metallicity via transmission spectroscopy,
for which WASP-107b is a very good target.
