Perihelion

The"werewolf curse" and "vampirism" are not magical afflictions, but rare, recessive alien biologies that lay dormant in human DNA. They were never meant to activate simultaneously. Soren, a reclusive astronomer, is a vampire whose sensitivity to sunlight is offset by an obsessive draw to study it. Lena, a wildlife biologist, is a werewolf whose connection to her wolf is weakening. During a rare solar eclipse, they are both caught in the same anomalous energy pulse from a deep-space phenomenon Soren was monitoring.
The event triggers a catastrophic merger: Soren now transforms under the full moon (burning under its light, not the sun), and Lena is now forced into a dormant, blood-hungry state under the sun. Their cycles are inverted and tied to the celestial bodies they once loved. To survive, they must spend two weeks together in dark seclusion (her vampire phase), and two weeks together in moonlit wilds (his wolf phase), constantly teaching the other how to navigate their stolen nature.
The romance is a desperate race against time and biology. They must learn to synchronize not just their hearts, but their very cellular rhythms, all while hunting for a cosmic cure before the next planetary alignment makes their condition permanent. They fall in love not as a vampire and a werewolf, but as two people sharing the same terrifying, beautiful curse.