When Yom Tov falls on Friday, one must prepare an Eruv Tavshilin before sunset on Erev Yom Tov to allow cooking on Yom Tov for Shabbat. The question was raised as to whether or not somebody who forgot to prepare an Eruv Tavshilin before sundown on Erev Yom Tov has the opportunity to do so after sunset.
Chacham Ovadia Yosef addresses this question in his work Yechaveh Da'at (6:31), and he concludes that in such a case one may, in fact, prepare his Eruv Tavshilin during the thirteen-minute period after sunset called Bein Ha'shemashot. He adds that one may even recite the Beracha when preparing the Eruv Tavshilin during this period.
Chacham Bentzion Abba Shaul (Jerusalem, 1924-1998), in Or Le'tziyon (22:8), adopts this position, as well. He adds, however, that once a person recites Arvit on the night of Yom Tov, or even responds to "Barechu" at the beginning of Arvit, he has effectively begun his observance of Yom Tov and therefore can no longer prepare an Eruv Tavshilin. Even if one responds to "Barechu" within thirteen minutes after sundown, he may no longer prepare the Eruv Tavshilin.
Summary: One who forgot to prepare an Eruv Tavshilin before sundown on Erev Yom Tov may still do so with a Beracha within thirteen minutes after sundown, unless he recites Arvit or responds to "Barechu" during Arvit, in which case he may no longer prepare the Eruv Tavshilin.
