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We report the discovery of two new transiting extrasolar planets. HAT-P-18borbits the V=12.759 K2 dwarf star GSC 2594-00646, with a periodP=5.508023+-0.6 d, transit epoch Tc=2454715.02174+-0.00020 (BJD), andtransit duration 0.1131+-0.0009 d. The host star has a mass of 0.77+-0.03 Msun,radius of 0.75+-0.04 Rsun, effective temperature 4803+-80 K, and metallicity[Fe/H]=+0.10+-0.08. The planetary companion has a mass of 0.197+-0.013 Mjup,and radius of 0.995+-0.052 Rjup yielding a mean density of 0.25+-0.04 g cm-3.HAT-P-19b orbits the V=12.901 K1 dwarf star GSC 2283-00589, with a periodP=4.008778+-0.6 d, transit epoch Tc=2455091.53417+-0.00034 (BJD), andtransit duration 0.1182+-0.0014 d. The host star has a mass of 0.84+-0.04 Msun,radius of 0.82+-0.05 Rsun, effective temperature 4990+-130 K, and metallicity[Fe/H]=+0.23+-0.08. The planetary companion has a mass of 0.292+-0.018 Mjup,and radius of 1.132+-0.072 Rjup yielding a mean density of 0.25+-0.04 g cm-3.The radial velocity residuals for HAT-P-19 exhibit a linear trend in time,which indicates the presence of a third body in the system. Comparing theseobservations with theoretical models, we find that HAT-P-18b and HAT-P-19b areeach consistent with a hydrogen-helium dominated gas giant planet withnegligible core mass. HAT-P-18b and HAT-P-19b join HAT-P-12b and WASP-21b in anemerging group of low-density Saturn-mass planets, with negligible inferredcore masses. However, unlike HAT-P-12b and WASP-21b, both HAT-P-18b andHAT-P-19b orbit stars with super-solar metallicity. This calls into questionthe heretofore suggestive correlation between the inferred core mass and hoststar metallicity for Saturn-mass planets.

HAT-P-18b and HAT-P-19b: Two Low-Density Saturn-Mass Planets Transiting Metal-Rich K Stars