Abstract: Ratios of cross sections, ?(Z+c??jets)/?(Z+jets), ?(Z+b??jets)/?(Z+jets), and ?(Z+c??jets)/?(Z+b??jets) in the associated production of a Z boson with at least one charm or bottom quark jet are measured in proton-proton collisions at ?s=13?TeV. The data sample, collected by the CMS experiment at the CERN LHC, corresponds to an integrated luminosity of 35.9?fb-1, with a fiducial volume of pT>30?GeV and |?|<2.4 for the jets, where pT and ? represent transverse momentum and pseudorapidity, respectively. The Z boson candidates come from leptonic decays into electrons or muons with pT>25?GeV and |?|<2.4, and the dilepton mass satisfies 71
