Severi curves of rational elliptic surfaces

We study Severi curves parametrizing rational bisections of elliptic fibrations associated to general pencils of plane cubics. Our main results show that these Severi curves are connected and reduced, and we give an upper bound on their geometric genus using quasi-modular forms. We conjecture that these Severi curves are eventually reducible, and we formulate a precise conjecture for their degrees in ${\mathbb{P}}^2$, featuring a divisor sum formula for collision multiplicities of branch points.