Constraints on ultralight scalars from compact stars

Ultralight scalar fields with electromagnetic and electrophilic
couplings to Standard Model particles can be sourced by compact stars,
giving rise to long-range interactions that mediate forces between stars
in binary systems. In certain scenarios, these scalars can also be
radiated from the stars, leading to observable effects. I will present
constraints on such ultralight scalars using measurements of pulsar
spin-down rates, orbital period loss in binaries, and characteristic
parameters of pulsars, considering the Crab pulsar, PSR J0737-3039, etc.
Additionally, I will discuss how the presence of a cosmic neutrino
background can quench these constraints. The constraints can be further
strengthened by several orders of magnitude with future radio observations
and precision measurements.