observation number

[Janusz Borkowski <borek@igf.edu.pl>]

Dear S+ users,
I have a data frame in which the first column indicates the day number, the 
second column  observation number and the third results of observations. It may 
look like:
Dn      On      R1
1       1       3
1       2       4
1       3       5
2       4       6
2       5       7
2       6       8
To find minimum value of R1 for each day is easy:
min.valR1<-Tapply(R1,Dn,min)
but how can I get not only the vector of minimum values, but also a vector of observation numbers for which the minimum occurred 
I am using S+ 4.5 for Windows.
I will appreciate any advice.
Janusz Borkowski