<qa>  What is the effect of letrozole and ribociclib?This phase II trial studies how well ribociclib and letrozole work in treating patients with estrogen receptor ER positive ovarian, fallopian tube, primary peritoneal, or endometrial cancer that has returned come back after a period of improvement. Ribociclib may stop the growth of tumor cells by blocking some enzymes needed for cell growth. Cancer cells that are estrogen receptor positive may need estrogen to grow. Letrozole lowers the amount of estrogen made by the body and this may stop the growth of tumor cells that need estrogen to grow. Giving ribociclib together with letrozole may be an effective treatment in patients with ovarian, fallopian tube, primary peritoneal, or endometrial cancer. 