Tight bounding box around labels in a textGrob object

how can i create a tight bounding box around labels in a textGrob object?

library(grid)
grid.newpage()
tg <- textGrob(
  strrep('R',1:3),
  seq(0.25,0.75,0.25),
  rep(0.5,3),
  rot = 0,
  just = c(0.5, 0.5),
  gp = NULL,
  name = "text"
)

grid.draw(tg)