Study of the doubly Cabibbo-suppressed decay D+--> phiK+ and the singly Cabibbo-suppressed decay Ds+--> phiK+

We have searched for the doubly Cabibbo-suppressed decay {ital D}{sup {plus minus}}{r arrow}{phi} {ital K}{sup {plus minus}} and the singly Cabibbo-suppressed decay {ital D}{sub {ital s}}{sup {plus minus}}{r arrow}{phi} {ital K}{sup {plus minus}} in data from the Fermilab photoproduction experiment E691. The {ital D}{sup {plus minus}} decay mode is of particular interest because it cannot result from simple spectator decay. We observe a {ital D}{sup {plus minus}} signal corresponding to a branching ratio of {ital B}({ital D}{sup {plus minus}}{r arrow}{phi}{ital K}{sup {plus minus}})=(3.3{sub {minus}1.5}{sup +1.8}{plus minus}0.8){times}10{sup {minus}4}. In the {ital D}{sub {ital s}}{sup {plus minus}} mode we measure an upper limit {ital B}({ital D}{sub {ital s}}{sup {plus minus}}{r arrow}{phi}{ital K}{sup {plus minus}}){lt}0.2%.