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We calculate the effects of a light, very weakly-coupled boson $X$ arisingfrom a spontaneously broken $U(1)_{B-L}$ symmetry on $\Delta N_{\rm eff}$ asmeasured by the CMB and $Y_p$ from BBN. Our focus is the mass range $1 \; {\rmeV} \lesssim m_X \lesssim 100 \; {\rm MeV}$; masses lighter than about an ${\rmeV}$ have strong constraints from fifth-force law constraints, while massesheavier than about $100$~MeV are constrained by other probes. We do not assume$X$ began in thermal equilibrium with the SM; instead, we allow $X$ tofreeze-in from its very weak interactions with the SM. We find $U(1)_{B-L}$ ismore strongly constrained by $\Delta N_{\rm eff}$ than previously considered.The bounds arise from the energy density in electrons and neutrinos slowlysiphoned off into $X$ bosons, which become nonrelativistic, redshift as matter,and then decay, dumping their slightly larger energy density back into the SMbath causing $\Delta N_{\rm eff} > 0$. While some of the parameter space hascomplementary constraints from stellar cooling, supernova emission, andterrestrial experiments, we find future CMB observatories can access regions ofmass and coupling space not probed by any other method. In gauging$U(1)_{B-L}$, we assume the $[U(1)_{B-L}]^3$ anomaly is canceled byright-handed neutrinos, and so our $\Delta N_{\rm eff}$ calculations have beencarried out in two scenarios: neutrinos have Dirac masses, or, right-handedneutrinos acquire Majorana masses. In the latter scenario, we comment on theadditional implications of thermalized right-handed neutrinos decaying duringBBN. We also briefly consider the possibility that $X$ decays into dark sectorstates. If these states behave as radiation, we find weaker constraints,whereas if they are massive, there are stronger constraints, though now from$\Delta N_{\rm eff} < 0$.

Cosmological Implications of Gauged $U(1)_{B-L}$ on $\Delta N_{\rm eff}$ in the CMB and BBN