WASP-63 or Kosjenka, also known as CD-38 2551, is a single star with an exoplanetary companion in the southern constellation of Columba. It is too faint to be visible with the naked eye, having an apparent visual magnitude of 11.1.^[2] The distance to this system is approximately 942 light-years (289 parsecs) based on parallax measurements, but it is drifting closer with a radial velocity of −24 km/s.

The designation WASP-63 indicates that this was the 63rd star found to have a planet by the Wide Angle Search for Planets.

In August 2022, this planetary system was included among 20 systems to be named by the third NameExoWorlds project.^[8] The approved names, proposed by a team from Croatia, were announced in June 2023. WASP-63 is named Kosjenka and its planet is named Regoč, after characters from Croatian Tales of Long Ago by Ivana Brlić-Mažuranić.^[9]

This is a G-type star with a stellar classification of G8;^[3] the luminosity class is currently unknown. The star is much older than the Sun at approximately 8.3^+1.3
_−1.2 billion years. WASP-63 is slightly enriched in heavy elements, having 120% of the solar abundance of iron.^[5] The stellar radius is enlarged for a G8 star,^[3] and models suggest it has evolved into a subgiant star.^[1] It has 1.1 times the mass of the Sun and is spinning with a projected rotational velocity of 3 km/s.^[5]

In 2012 a transiting gas giant planet WASP-63b was detected on a tight, circular orbit.^[3] Its equilibrium temperature is 1536±37 K, and measured dayside temperature is 1547±308 K.^[10] The planet is similar to Saturn in mass but is highly inflated due to proximity to the parent star. The planetary atmosphere contains water and likely has a high cloud deck of indeterminate composition.^[11]