The Harris-Venkatesh conjecture for derived Hecke operators II: a unified Stark conjecture

We prove a compatibility theorem between the Stark conjecture and the Harris-Venkatesh conjecture for imaginary dihedral modular forms of weight $1$. The key technical input is a general two-variable $\mathrm{PGL}_2$ Siegel-Weil formula that precisely gives the Laurent series coefficients of Eisenstein series as a dual theta lift of Eisenstein series. This two-variable Siegel-Weil formula is applied to Rankin-Selberg periods of imaginary dihedral optimal forms and a refinement of Stark's formula relating $L$-values with elliptic units, yielding compatibility between derived Hecke operators and adjoint $L$-values for Deligne-Serre representations. We formulate a general unified conjecture encompassing the Stark and Harris-Venkatesh conjectures and conceptually reinterpret the action of derived Hecke operators as modular Stark unit data at almost all primes.