Structure functions, as measured in lepton-nucleon scattering, have proven to be very useful in studying the partonic dynamics within the nucleon. However, it is experimentally difficult to separately determine the longitudinal and transverse structure functions, and consequently there are substantially less data available in particular for the longitudinal structure function. Here, we present separated structure functions for hydrogen and deuterium at low four-momentum transfer squared, Q 2 < 1 GeV 2 , and compare them with parton distribution parametrization and k T factorization approaches. While differences are found, the parametrizations generally agree with the data, even at the very low-Q 2 scale of the data. The deuterium data show a smaller longitudinal structure function and a smaller ratio of longitudinal to transverse cross section, R, than the proton. This suggests 2469-9985/2018/97(4)/045204 (11) 045204-1 ©2018 American Physical Society V. TVASKIS et al.PHYSICAL REVIEW C 97, 045204 (2018) either an unexpected difference in R for the proton and the neutron or a suppression of the gluonic distribution in nuclei.