[R] merge xts objects with different data types ?

[ce]

Hello

Let's say some questions about merging  xts variables :

a<- xts("abc", Sys.Date())
b <- xts("def", Sys.Date())
c <- xts(1, Sys.Date())

> merge(a,b)
           a     b    
2015-09-03 "abc" "def"
> merge(a,b,c)
            a  b c
2015-09-03 NA NA 1
Warning messages:
1: In merge.xts(a, b, c) : NAs introduced by coercion
2: In merge.xts(a, b, c) : NAs introduced by coercion
3: In merge.xts(a, b, c) : NAs introduced by coercion
4: In merge.xts(a, b, c) : NAs introduced by coercion

How I can merge  a, b ,c correctly ? Another example is with Binary variables :

> e<- xts(TRUE, Sys.Date())
> e
           [,1]
2015-09-03 TRUE
> merge(e,b)
           e  b
2015-09-03 1 NA
Warning message:
In merge.xts(e, b) : NAs introduced by coercion


My second question is how I can convert an xts object to factor :

> d <- merge(a,b)
> d
           a     b    
2015-09-03 "abc" "def"
> factor(d, levels = c("abc","def"))
  a   b 
abc def 
Levels: abc def

Date disappears here?

Thanks for your help
ce