The last U.S. zoo with pandas in its care expects to say goodbye to the four giant bears this fall. Zoo Atlanta is making preparations to return panda parents Lun Lun and Yang Yang to China along with their American-born twins Ya Lun and Xi Lun, zoo officials said Friday. There is no specific date for the transfer yet, they said, but it will likely happen between October and December.The four Atlanta pandas have been the last in the United States since the National Zoo in Washington returned three pandas to China last November. Other American zoos have sent pandas back to China as loan agreements lapsed amid heightened diplomatic tensions between the two nations.Atlanta received Lun Lun and Yang Yang from China in 1999 as part of a 25-year loan agreement that will soon expire. Ya Lun and Xi Lun, born in 2016, are the youngest of seven pandas born at Zoo Atlanta since their parents arrived. Their siblings are already in the care of China's Chengdu Research Center of Giant Panda Breeding.It is possible that America will welcome a new panda pair before the Atlanta bears depart. The San Diego Zoo said last month that staff members recently traveled to China to meet pandas Yun Chuan and Xin Bao, which could arrive in California as soon as this summer.Zoo Atlanta officials said in a news release they should be able to share "significant advance notice" before their pandas leave. As to whether Atlanta might see host any future pandas, "no discussions have yet taken place with partners in China," zoo officials said.