<P> Michael Lembeck joined the series as Julie's husband, Max, in the fifth season, but he was written out as a consequence of Phillips' firing (but later returned in season seven along with Philips). A steady stream of regulars was added in the ensuing seasons, including Ron Rifkin as Ann's boyfriend, Nick; Glenn Scarpelli as Nick's son, Alex; and Boyd Gaines as Barbara's boyfriend, later husband, Mark . Shelley Fabares, who had previously guest - starred as Ann's rival co-worker Francine Webster, appeared more frequently, eventually becoming a regular . Nanette Fabray, who played Ann's mother, also made more frequent appearances before becoming a regular cast member in the final season . Howard Hesseman joined the series for a short time as Mark's father, Sam, who would become Ann's second husband . </P> <P> Notable guest stars throughout the series run include Norman Alden, Robby Benson, Carla Borelli, Charlie Brill, Dennis Burkley, Jack Dodson, Elinor Donahue, David Dukes, Greg Evigan, Conchata Ferrell, Corey Feldman, Alice Ghostley, Lee Grant, Mark Hamill, Jim Hutton, Terry Kiser, Richard Kline, Christopher Knight, Jay Leno, Robert Mandan, Robert Morse, Denise Nicholas, J. Pat O'Malley, Jo Ann Pflug, Eve Plumb, Susan Richardson, William Schallert, Gretchen Corbett, Suzanne Somers, Ellen Travolta, Dick Van Patten, and Keenan Wynn . </P> <Table> <Tr> <Th> Actor </Th> <Th> Character </Th> <Th_colspan="9"> Seasons </Th> </Tr> <Tr> <Th> </Th> <Th> </Th> <Th> </Th> <Th> </Th> <Th> 5 </Th> <Th> 6 </Th> <Th> 7 </Th> <Th> 8 </Th> <Th> 9 </Th> </Tr> <Tr> <Td> Bonnie Franklin </Td> <Td> Ann Romano </Td> <Td_colspan="9"> Main </Td> </Tr> <Tr> <Td> Mackenzie Phillips </Td> <Td> Julie Cooper Horvath </Td> <Td_colspan="5"> Main </Td> <Td> </Td> <Td_colspan="3"> Recurring </Td> </Tr> <Tr> <Td> Richard Masur </Td> <Td> David Kane </Td> <Td_colspan="2"> Main </Td> <Td_colspan="3"> </Td> <Td> Guest </Td> <Td_colspan="3"> </Td> </Tr> <Tr> <Td> Valerie Bertinelli </Td> <Td> Barbara Cooper Royer </Td> <Td_colspan="9"> Main </Td> </Tr> <Tr> <Td> Pat Harrington, Jr . </Td> <Td> Dwayne Schneider </Td> <Td_colspan="9"> Main </Td> </Tr> <Tr> <Td> Mary Louise Wilson </Td> <Td> Ginny Wroblicki </Td> <Td> </Td> <Td> Main </Td> <Td_colspan="7"> </Td> </Tr> <Tr> <Td> Michael Lembeck </Td> <Td> Max Horvath </Td> <Td_colspan="4"> </Td> <Td> Main </Td> <Td> </Td> <Td_colspan="2"> Recurring </Td> <Td> Main </Td> </Tr> <Tr> <Td> Ron Rifkin </Td> <Td> Nick Handris </Td> <Td_colspan="5"> </Td> <Td_colspan="1"> Main </Td> <Td_colspan="3"> </Td> </Tr> <Tr> <Td> Glenn Scarpelli </Td> <Td> Alex Handris </Td> <Td_colspan="5"> </Td> <Td_colspan="3"> Main </Td> <Td> </Td> </Tr> <Tr> <Td> Boyd Gaines </Td> <Td> Mark Royer </Td> <Td_colspan="6"> </Td> <Td> Recurring </Td> <Td_colspan="2"> Main </Td> </Tr> <Tr> <Td> Shelley Fabares </Td> <Td> Francine Webster </Td> <Td_colspan="2"> </Td> <Td_colspan="2"> Guest </Td> <Td> </Td> <Td> Guest </Td> <Td> Recurring </Td> <Td> Main </Td> <Td> Recurring </Td> </Tr> <Tr> <Td> Nanette Fabray </Td> <Td> Katherine Romano </Td> <Td_colspan="3"> </Td> <Td> Guest </Td> <Td_colspan="4"> Recurring </Td> <Td> Main </Td> </Tr> <Tr> <Td> Howard Hesseman </Td> <Td> Sam Royer </Td> <Td_colspan="7"> </Td> <Td> Recurring </Td> <Td> Main </Td> </Tr> </Table> <Tr> <Th> Actor </Th> <Th> Character </Th> <Th_colspan="9"> Seasons </Th> </Tr>

Who played mark on one day at a time