A grow-by-one object is a pattern whose population increases by one cell every generation. The smallest known grow-by-one object is the following 44-cell pattern (David Bell's one-cell improvement of a pattern found by Nicolay Beluchenko, September 2005):
#O David Bell, Nicolay Beluchenko, September 2005
#C A 44-cell grow-by-one-object; the smallest such object known as of March 2018.
#C htt://conwaylife.com/wiki/Grow-by-one_object
x = 17, y = 15, rule = B3/S23
8b2o$7b2o$9bo$11b2o$10bo2$9bo2b2o$b2o5b2o4bo$2o5bo5bo$2bo4bobo3b2o$4bo
2bo4b2obo$4b2o7b2o$8bo4bob2o$7bobo2bob2o$8bo!
#C [[ THUMBSIZE 2 THEME 6 GRID GRIDMAJOR 0 SUPPRESS THUMBLAUNCH ]]
#C [[ ZOOM 16 THUMBSIZE 2 WIDTH 640 HEIGHT 640 ]]