HATS-37Ab and HATS-38b: Two transiting hot neptunes in the desert^*

We report the discovery of two transiting Neptunes by the HATSouth survey. The planet HATS-37Ab has a mass of 0.099 ± 0.042 M_J (31.5 ± 13.4 M) and a radius of 0.606 ± 0.016 R_J, and is on a P = 4.3315 day orbit around a V = 12.266 ± 0.030 mag, 0.843_-⁺_0.012^0.017 M_☉ star with a radius of 0.877_-⁺_0.012^0.019 R_☉. We also present evidence that the star HATS-37A has an unresolved stellar companion HATS-37B, with a photometrically estimated mass of 0.654 ± 0.033 M_☉. The planet HATS-38b has a mass of 0.074 ± 0.011 M_J (23.5 ± 3.5 M) and a radius of 0.614 ± 0.017 R_J, and is on a P = 4.3750 day orbit around a V = 12.411 ± 0.030 mag, 0.890_-⁺_0.012^0.016 M_☉ star with a radius of 1.105 ± 0.016 R_☉. Both systems appear to be old, with isochrone-based ages of 11.46_-⁺_1.45^0.79 Gyr, and 11.89 ± 0.60 Gyr, respectively. Both HATS-37Ab and HATS-38b lie in the Neptune desert and are thus examples of a population with a low occurrence rate. They are also among the lowest-mass planets found from ground-based wide-field surveys to date.