Temporary Marriage 

When she and her sister are unexpectedly orphaned and prevented from traveling West unless they have a male chaperone, Mattie Prescott disguises herself as a boy. But after Mattie's fellow wagon train companions discover her masquerade, their long-dreamed-of fresh start is over before it's even begun. She has only one choice: marriage to the man who helped her—and kept her secret—along the trail. 

To save her honor and his own, Josiah Dawson agrees to take Mattie as his bride. But his plans don't include a wife, and this hasty union is just a temporary duty he'll dissolve at the end of the trail. As Mattie proves herself indispensable in the face of tragedy, though, it's soon clear that she's also indispensable to Josiah's heart…